Conjugated diene polymer cyclized products obtained by introducing a ring structure into a conjugated diene polymer rubber such as natural rubbers and synthetic polyisoprene rubbers have been employed as a photoresist material. A conjugated diene polymer cyclized product used for this application is required of a low metal content, an adequate molecular weight and a narrow molecular weight distribution.
In order to meet such requirements, Patent Document 1 discloses a method for producing a high-purity diene polymer cyclized product, which is characterized by polymerizing a diene monomer by using an organometal compound represented by the general formula: RM (wherein R represents an alkyl group, an aryl group, or the like; and M represents an element of the Group I of the periodic table), subsequently cyclizing the polymer by a specific fluorine-containing substituted sulfonic acid compound, and then washing the reaction liquid with water. According to this method, it is described that the cyclization reaction can be achieved with high activity without generating a gel and that a high-purity diene polymer cyclized product having a very low metal ion content can be obtained in a good yield without complicated post-treatments.
Patent Document 2 discloses that an isoprene polymer cyclized product having a degree of dispersion ((weight average molecular weight)/(number average molecular weight)) of not more than 2 is obtained by polymerizing an isoprene monomer in the presence of an organometal represented by the general formula: RM (wherein R represents an alkyl, aryl or aralkyl group; and M represents lithium, sodium or potassium) and cyclizing this polymer by a catalyst composed of a boron trifluoride ether complex and a specific carboxylic acid.
Patent Document 3 discloses a method for producing a conjugated diene polymer cyclized product by adding a phenolic compound to a conjugated diene polymer solution, obtained by using an organolithium compound, and making a Lewis acid and a Broensted acid act in this conjugated diene polymer solution. According to this method, it is described that by suppressing broadening of the molecular weight distribution of the conjugated diene polymer cyclized product, a cyclized product having a molecular weight distribution of less than 2, and preferably not more than 1.5 is obtainable.    Patent Document 1: JP-A-57-070102    Patent Document 2: JP-A-59-096112    Patent Document 3: JP-A-61-163904
Now, in recent years, it has been found that conjugated diene polymer cyclic products are not only excellent in chemical resistance, heat resistance and weather resistance but also excellent in adhesion properties to non-polar polymers such as polyolefins; polar polymers such as polyesters, polyurethanes, and alkyd resins; metals such as iron; and so on. For that reason, the conjugated diene polymer cyclized products have been being applied for various applications including paints, adhesives, electric or electronic materials, vehicles for printing inks, and optical materials, besides applications related to resists for which they have hitherto been praisefully used.
A conjugated diene polymer cyclized product which is used for these applications, especially applications such as electronic materials, vehicles for printing inks, primers for paints, optical materials, and resists is required to be free from coloring in addition to the narrow molecular weight distribution.
The present inventor studied the methods as described in the above-mentioned respective patent documents, and have understood that, aside from the molecular weight distribution, coloring of the resulting polymer is so considerable that the polymer does not reach levels required for the above-mentioned applications at all.